[PATCH] [MachineSink] Use the real post dominator tree

[Jingyue Wu]

I was trying to rebase this diff against the latest version. However, one test case (test/CodeGen/X86/loop-strength-reduce8.ll) failed. The failure is not directly caused by this change in machine sinking but triggered by it. I reduced the test case and filed PR20701 (http://llvm.org/bugs/show_bug.cgi?id=20701). It would be great if someone here can take a look. Many thanks!

http://reviews.llvm.org/D4814